Orogenic collapse

In geology, orogenic collapse is a fascinating process where thickened parts of the Earth's crust become thinner and spread out sideways. This happens because the thick crust has a lot of gravitational potential energy, and it naturally moves to areas with less energy. Orogenic collapse can start at any time during an orogeny, which is when mountains are being built up by the movement of tectonic forces.

After the main mountain-building events stop, we call this process post-orogenic collapse or post-orogenic extension. These processes are important because they mark a key stage in the Wilson Cycle, the cycle of how continents move, collide, and then eventually break apart again. Understanding orogenic collapse helps scientists learn more about how Earth's surface changes over millions of years.

Description

Orogens, also called mountain ranges, form when tectonic plates crash into each other, piling up the Earth's crust and making it thicker. This thickening is the beginning of an orogeny, a time when mountains are built. But as the mountains grow, the thick crust can become unstable and start to spread out and thin — a process called orogenic collapse.

Two main things cause this collapse. First, the heavy, thick crust has a lot of gravity pulling it down, making it break and spread out under its own weight. Second, the thick crust sinks into the Earth’s mantle, where heat makes the rock softer and easier to move. This heat lets material from deeper down rise into thinner areas, helping to reduce the thickness. While erosion and eduction can also wear down mountains, orogenic collapse is a key way mountains can lower themselves.

Models

A fixed-boundary collapse happens when the Earth's outer layer becomes too thick while forces from plate movements are still at work. This can bring up parts of the Earth that were buried deep down.

Free-boundary collapse occurs when the forces pushing the Earth's layers stop, allowing the thick crust to move freely. This causes the surface to stretch and the deeper layers to flow toward thinner areas. The stretching can make cracks form on the surface, similar to how a piece of Camembert cheese might sag and split if left out overnight. This process involves extension of the surface and normal faulting.

Examples

Caledonian orogeny

Main article: Caledonian orogeny

The Scandinavian Caledonides is an example of a mountain chain that once reached heights of 8–9 km before it thinned and spread out during the Devonian. This process formed major structures such as the Nordfjord-Sogn Detachment. Today, the height of the Scandinavian Mountains comes from processes that happened much later, during the Cenozoic.

Basin and Range Province

The Basin and Range Province in the western United States was once a high plateau. Over time, it stretched and thinned, breaking into blocks of land called fault blocks. Scientists debate why this happened, but it may be linked to changes in the movement of the Earth's plates, including the shift from a subduction zone to a transform boundary between the North American and Pacific plates.

Aegean Sea Plate

The Aegean Sea Plate is a part of the Earth's crust that has thinned over time. It lies between the Mediterranean and the Black Sea. After a period of mountain-building, the crust thinned further due to movements deep within the Earth involving the African Plate.

Variscan orogeny

See also: Variscan chain and Variscan orogeny

The Variscan orogeny happened when two large pieces of Earth's crust, Laurussia and Gondwana, collided to form Pangaea. This created a thick, high plateau. Later, the thick crust began to thin due to natural processes, including the movement of deep Earth layers.